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Commercial stroboscopic discs sometimes carry three rings of 76, 77 and 78 bars. These correspond to speeds of 78-9, 77-9 and 76-9 r.p.m. respectively. Do not be misled by the purely coincidental similarity in this case between the number of bars and the r.p.m. A disc for 33J r.p.m. requires 180 bars.
When you are changing records frequently, it is inconvenient to keep putting a stroboscopic disc on the record, only to remove it later. You can always make an immediate speed check, by marking 77 bars around the turntable rim. This you can do most readily by painting the bars on a piece of paper of the right length and then sticking this round the rim. Be sure that the bars are of even thickness and uniformly spaced.
Some commercial turntables already carry stroboscopic bars around their rim.
Types of Motor
If the turntable motors are of the synchronous type, then the speed at which they run is determined entirely by the frequency of the electricity supply. If the motor drives the turntable directly, or through gearing, the turntable speed will also depend on the mains frequency. You can check whether your turntable is truly synchronous by using a stroboscopic disc (p. 21). One set of bars should show absolutely no progressive creep either forward or backwards. There will probably be some swaying of the pattern, due to slight eccentricity, but you can ignore this. Variations in mains voltage, or in stiffness of the lubricating grease, or the drag of the pick-up, will not affect the turntable speed. Using a pair of such turntables, you will never have to worry about the possibility of a speed difference between consecutive records.
If the pattern on a stroboscopic disc always shows a little slip one way or the other, the turntable is probably driven by an induction motor. Most modern electric turntables use
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